Method of substituting one ferrofluid solvent for another



sePf- 29, 1970 R. E. ROSENSWEIG 3,531,413

METHOD OF SUBSTITUTING ONE FERROFLUID SOLVENT FOR ANOTHER Filed sept.22, 1967 2 sheets-sheet 1 G ENERGY ff E a PARTncLE SEPARATION PARTCLEATTRACTING ENERGY REPELLIN (VAN DER WAAL) ATTRACTING ENERGY REPELLINGENERGY VAN DER WAAL) INVENTOR.

RONALD E. ROSENSWEIG sept 29, 1970 l R. E. RosENswElcsA 3,531,413

METHOD OF SUBSTITUTING ONE FERROFLUID SOLVENT FOR ANOTHER Filed sept.22, 1967 2 sheets-sheet 2 INVE R. RONALD E. ROSE WEIG United StatesPatent @ii 3,53lAl3 Patented Sept. 29, 1970 3,531,4ll3 METHD FSUBSTH'EUTENG @NE lFlERlRUFLlUlD SULVENT FR ANG'I'HER Ronald lE.Rosensweig, Lexington, Mass., assigner to Aveo Corporation, Cincinnati,Unio, a corporation of Delaware Filed Sept. 22, 1967, Ser. No. 669,938int. Cl. Bllld 2]/0] US. Cl. 252-62.62 6 Claims ABSTRACT @E THEDISCLOSURE This invention covers a method of substituting one ferroiluidsolvent for another. A typical class of ferrofluid contains magneticparticles with a polar surfactant adsorbed on the surface thereofsuspended in a non-polar solvent. To substitute one solvent for anotherin accordance with the invention, a polar ilocculating solvent isintroduced into the ferrofluid causing the particles with the adsorbedsurfactant to tlocculate, and to settle out of solution.

The particles and solvent are separated. The particles are thenredistributed within another solvent which may be the same as theoriginal solvent or a different composition.

BACKGROUND OF THE INVENTION The invention relates to ferrouid and moreparticularly to methods of making ferrouids. Related information may befound in Pat. No. 3,281,403.

In general, ferrofluids contain submicron particles of magnetic materialsuch as magnetite and ferrites. A surfactant or dispersing agent isadsorbed on the surface of these particles and acts as a coupling agent`between the particle and a solvent in which the particles aredispersed. Additional information on the structure of typicalferrolluids are found in an article entitled Magnetic Fluids by R. E.Rosensweig in the July 1966 issue of International Science andTechnology.

Ferrotluids are generally made by grinding magnetic particles togetherwith a surfactant and solvent over a prolonged period of time.

Not all surfactants or solvents are suitable grinding media. At timesthey are totally unsuitable and other times ineilicient.

Examples of magnetic material which are suitable for this purpose aremagnetite (.FE3O4), manganese-zinc ferrite, v-FezOa, and any solidmagnetic material that can be `formed in the proper particle size.Surfactants of the following general nature have been used to makeferrolluids. Carboxylic acids and their metallic salts, amines, succinicacid derivatives, condensation product of an amino ester of a fattyacid. The chain length should usually be eight carbon atoms or greater.Examples of solvents are aliphatic hydrocarbons such as heptane, octane,decane, mineral oil, kerosene etc., halogenated hydrocarbons such ascarbon tetrachloride or trichlorethylene and aromatic solvents such asbenzene, toluene and their non-polar derivatives. All of these solventsare suitable for grinding and are interchangeable.

Pentane is a suitable solvent but not useful as a grinding aid becauseit is very volatile.

Flocculation is used herein to denote the aggregation of individualparticles into larger masses. These aggregations eventually grow largeenough to deposit out of suspension.

It is an object of the invention to provide a method of substituting onesolvent of a ferrofluid by another.

It is an object of the invention to provide a method of substituting onesolvent of a ferroiluid by another by inducing particle flocculation tocause the magnetic particle to deposit out of solution.

It is another object of the invention to provide a means for formulatingferrolluids containing solvents that are not suitable as grinding ormanufacturing aids.

It is yet another object of the invention to provide means for alteringthe particle concentration in a ferroiiuid particularly Where aferrofluid surfactant is more volatile than the solvent.

It is still another object of the invention to induce flocculation forthe purpose of substituting solvents by reducing the repelling forcesoverriding the Van der Waal forces of attraction.

In accordance with the invention a method of substituting one ferrofluidsolvent for another ferrolluid comprises the steps of introducing aflocculation agent into the ferrofluid thereby causing the particleswith adsorbed surfactant to come out of suspension. The particles areseparated from the solvent and this is followed by resuspending them inanother solvent.

The novel features that are considered characteristic of the inventionare set forth in the appended claims; the invention itself, however,both as to its organization and method of operation, together withadditional objects and advantages thereof, will best be understood fromthe following description of a specic embodiment when read inconjunction with the accompanying drawings, in which:

FIG. 1 shows a curve useful in explaining the forces acting onferrofluid particles.

FIG. 2 is a schematic representation of a ferrouid structure.

FIG. 3 shows a second curve useful in explaining the forces acting onferrofluid particles.

FIG. 4 depicts the relationship of the various components of aferrofluid in which a flocculation solvent has been added.

FIG. 5 illustrates an alternate method of causing particles toflocculate.

Characteristically, magnetic particles in a ferrouid remain insuspension without changing the characteristics of the ferrofluid as ahomogeneous medium under the influence of applied magnetic fields andmagnetic field gradients. There are two contributing factors thatmaintain the magnetic particles in suspension. The rst relates to thesize of the particles which typically are in the submicron region sothat particle motion is maintained by thermal agitation. Secondly, thesurfactant 0r dispersing agent acts to maintain the particlessufliciently remote from one another to overcome the force of attractioncaused by Van der Waals forces.

The origin of the Van der Waal force is the attraction of a fluctuatingelectric dipole for a neighboring induced dipole. So long as thesurfaces of adjacent particles are about one radius apart, the particleswill not be strongly attracted toward each other and flocculation can beavoided. The function of the adsorbed coating or surfactant on theparticles is to provide at least the minimum distance required forseparation.

In FIG. 1 curve 21 represents the magnitude of Van der Waal energy as afunction of distance separating two particles. Curve 20 represents therepelling energy generated by coatings of adsorbed surfactant on thesurface of two adjacent particles. Curve 22 represents the algebraic sumof the attracting and repelling energies.

The repulsive force equals the negative rate of change of energy withdistance dE (Prin (assuming the convention that positive forcerepresents repulsion.) Clearly that repulsion occurs to the right of theapex 25 of curve 23 since the slope of the curve 23dE/dX or the rate ofchange of energy as a function of distance is negative.

Additionally, if the magnitude of energy at the apex exceeds the thermalenergy per particle, the thermal fluctuations of the particles will nowthrow the particles close enough together to come within the attractionregion, to the left of the apex.

Referring to FIG. 2 of the drawings, there is a schematic representationof two adjacent particles and 11 suspended within a non-polar solvent12. Absorbed on each surface of the particles 10 and 11 are molecules 13of a polar surfactant. It will Ibe noted that the molecules 13 contain apolar head 14 and a non-polar tail 15. The surfactant molecules 13 coatthe surface of the particles, and in effect, form as an elastic boundarybetween the particles. When two particles such as 10 and 11 approacheach other the coating is` compressed and provides an elastic repulsiongreater than the attractive forces that would otherwise cause particlesto come into contact until flocculation occurs and the solid materialsettles out.

Referring to FIG. 4 of the drawings, the particles 10 and 11 aredepicted in substantially the same environment as shown for FIG. 2,except for the fact that a occulating solvent depicted by molecules 16has been added to the ferrofluid. The flocculating solvent is a polarsolvent. Its effect on the ferroiiuid is to make the surfactantmolecules less compatible with its fluid environment comprising thecombination of ferrofluid solvent and lilocculating solvent. The tailsections of the surfactant molecules are repelled by the combination ofsolvents and tend to fold back on themselves toward the surface of theparticles. Thus, the depth of coating is materially reduced.

The curves in FIG. 3 denote, as before, Van der Waal attraction,surfactant repulsion and the algebraic sum. Energy curve 23 in FIG. 3reflects the shortened surfactant thickness as shown in FIG. 4. At nopoint does it go above the abscissa and therefore nowhere does therepulsion energy exceed the Van der Waal attraction energy.

Particles in their random movement through the combined solvent nolonger encounter a repelling force in excess of the Van lder Waalattraction force. The particles adhere to each other, flocculate anddeposit out of the combined solvent.

From this point it is a relatively simple matter to separate theparticles from the combined solvent. A little of the flocculating andferrofluid solvent remains on the particles. These solvents appear as avery small impurity when the particles are resuspended in the newsolvent. No problems are encountered by these impurities as they arecompatible with the new ferrofluid in these small quantities.

In the event the purpose in occulating the particles was to increase theconcentration, it is now clearly obvious that the one need only add areduced amount of solvent to the particles. On the other hand, if thepurpose in occulating the particles was to substitute one solvent foranother, this step is now clearly possible.

Typical formulations used in practicing the invention are provided inTable No. 1 which follows. The ferrofluid sCl/ents and flocculationsolvents are fully interchangea e.

Another means for causing flocculation, called polymeric flocculation,is shown in FIG. 5. Flocculation is caused by linking of two-ormore-particles 10, 11, 17

4 by a polymer molecule 18 as shown in FIG. 5. The particles that settleout of solution may be redispersed by supplying excess solvent to theparticles to dissolve some of the entangling species.

The amount of flocculating agent that is required differs with eachapplication. It is, however, determinable through routine procedures.

Observed iiocculating agents are polyisobutylene which works withaliphatic and chlorinated aliphatic solvents; polystyrene which may beused with aromatic solvents including toluene; and dimethylene siloxanepolymers with the aliphatics.

Two important criteria of polymer occulation agents are: (i) they bemiscible in the ferrouid solvent, and (ii) the molecule length shall beseveral times in the diameter of a particle since a linking of particlesis an important consideration.

CONCLUSION In its broadest aspect, the invention covers the substitutionof one ferrofluid solvent for another by introducing into the ferrofluida flocculating agent. The agent can reduce the forces acting inopposition to the Van der Waal forces to the extent thatA the particlescan be attracted to one another and deposit out of solution.Alternatively, the occulating agent can act to link particles causingocculation and ultimately the separation of particles and solvent. V

Secondarily, with respect to the former approach, the invention coversthe concept of shrinking the thickness of the surfactant coating thusenabling the Van der Waal forces to bring and maintain adjacentparticles together. Another aspect of the invention causes the coatingto shrink and reduce its effective thickness by introducing into aferrouid containing a polar surfactant and a nonpolar solvent, a polarflocculating agent.

The various features and advantages of the invention are thought to beclear from the foregoing description. Various other features andadvantages not specifically enumerated will undoubtedly occur to thoseversed in the art, as likewise Will many variations and modifications ofthe preferred embodiment illustrated, all of which may be achievedwithout departing from the spirit and scope of the invention as deiinedby the following claims.

What is claimed is:

1. A method of substituting one ferrouid solvent for another ferrouidsolvent in a ferrofluid consisting essentially of suspended magneticparticles, a polar surfactant and a non-polar ferrofluid solventcomprising the steps of z (a) introducing a polar flocculation agentinto the ferrofluid causing theiparticles with adsorbed surfactant toocculate and settle out of suspension;

(b) separating the original ferrofluid solvent and flocculation agentfrom the particles with adsorbed surfactant; and

(c) dispersing said occulated particles in another nonpolar ferrofluidsolvent.

2. A method as defined in claim 1 in which the surfactant is a longchain molecule having a chain length of at least eight carbon atomshaving a polar end group and a non-polar tail, said ferrofluid solventis a non-polar organic fluid and said flocculating agent is a polarorganic fluid soluble in the ferrofluid solvent.

3. A method as defined in claim 1 in which the surfactant is a member ofthe class consisting of carboxylic acids and their metallic salts,amines, succinic acid derivatives, condensation product of an aminoester of a fatty acid, said ferrouid solvent is a member of the classconsisting of aliphatic hydrocarbons, halogenated aliphatic hydrocarbonsand aromatic hydrocarbons, and said flocculating agent is a member ofthe class consisting of acetone, ethyl alcohol, dioxane, ethyl acetateand acetone.

4. A method of substituting one ferrouid solvent for another ferrofluidsolvent in a ferroiluid consisting essentially of magnetic particles asurfactant adsorbed on said 6 particles and a non-polar ferrofluidsolvent comprising the fluid solvent is an aromatic hydrocarbon and theoccusteps of: lating agent is polystyrene.

(a) introducing a flocculating agent which is a miscible polymerparticle linking agent having a 4molecular References Cited chain lengthat least twice the diameter of the mag- 5 UNITED STATES PATENTS netlcparticles mto the ferrouid and occulatlng sald 2 590 997 4/ 1952Mitchell 252-326 partlcles causmg said partlcles to settle out ofsuspension; 3,290,252 12/ 1966 Larsen 252-6254 (b) separating theferrofluid solvent and polymer link- FOREIGN PATENTS ing agent from theparticles with adsorbed surfactant; 10 974 627 11/1964 Great Britain and(c) dispersing said flocculated particles in another non- TOBIAS ELEVOW, Primary Examiner polar ferrouid solvent. 5. A method as definedin claim 4 in which the ferro- I' COOPER Assistant Exammer fluid solventis an aliphatic hydrocarbon and the tioc- 15 U S C1 XR culating agent isselected from the group consisting of polyisobutylene and dimethylsiloxane. 252-6251, 62,56, 309, 327

6, A method as dened in claim 4 in which the ferro- Patent No. 3 531:413 Dated September Z9, 1970 Inventor(s) Ronald E. Rosenweg It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

Column l, line 48, for "(FE O read{Fe O4) Column Z, line l5, after"ferroflud'read"solven Column 3, line 8, for "now" read not; Column 3,line 39, after "thickness" omitas; Column 3, line 57, after "that"omit--the.

SIGNED AND (l5-LED DEC im tSEAL) Attest:

M Flethg Ir- Eo y l :'i p nl L! OEE @omissionsor PatentaJ

